(1) Field of the Invention
The present application relates to a surface mountable over-current protection device, and more particularly to a surface mountable over-current protection device with high hold current and positive temperature coefficient (PTC) characteristics.
(2) Description of the Related Art
Because the resistance of conductive composite materials having PTC characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance instantaneously increases to a high resistance state (e.g., at least 102Ω), so as to suppress over-current and protect the cell or the circuit device.
In general, the PTC conductive composite contains at least one crystalline polymer and conductive filler. The conductive filler is dispersed uniformly in the crystalline polymer. The crystalline polymer is mainly a polyolefin polymer or a fluoropolyolefin polymer such as polyethylene, polyvinyl fluoride or polyvinylidene fluoride (PVDF). The conductive filler(s) is mainly carbon black.
The conductivity of the PTC conductive composite depends on the content and type of the conductive fillers. In general, the resistivity of the PTC conductive composite containing the carbon black as the conductive filler seldom reaches below 0.2 Ω-cm. Even though the low resistivity below 0.2 Ω-cm is achieved, the PTC conductive composite often loses the characteristic of voltage endurance. Therefore, a conductive filler, which is different from carbon black, with lower resistance should be used in the PTC conductive composite to reach a resistivity below 0.2 Ω-cm. The conductivity of carbon black is relatively low (i.e., relatively high resistance). If carbon black is applied to a surface mountable device (SMD) with fixed covered area, the hold current of the SMD is limited to certain level due to the resistance limitation of carbon black. The hold current indicates a maximum current that the PTC device can endure before trip at a specific temperature.
Although a multi-layer PTC structure could be used to increase the hold current, SMD over-current protection device performance is eventually limited due to the limitation of total height as well as the number of PTC layers of the SMD device. In general, for an SMD over-current protection device including a single PTC layer having carbon black, the ratio of the hold current to the area of a PTC material layer cannot exceed 0.16 A/mm2 The SMD over-current protection devices currently available in the market have a certain shape characterized by the width and the length, which are defined as a form factor in the specification. Consequently, the length and width of the SMD over-current protection device determine its covered area. For example, SMD 1812 indicates a SMD with a length of 0.18 inches and a width of 0.12 inches, and thus a covered area is equal to 0.18″×0.12″, which is equivalent to 4.572 mm×3.048 mm=13.9355 mm2 in metric system. For an over-current protection device of SMD 1812 using carbon black as the conductive filler, a single PTC material layer hardly reaches a hold current of 1.8 A. If the SMD 1812 having two PTC material layers can hold a current up to 3.6 A, the hold current per unit covered area of a single PTC material layer can be calculated as: 3.6 A/(2×13.9355 mm2)=0.129 A/mm2, which is below 0.16 A/mm2. Therefore, it is highly desirable that a new type SMD device could be developed to exceed the 0.16 A/mm2 barrier.
U.S. Pat. No. 8,044,763 disclosed the use of conductive filler with low resistivity such as metal powder or metal carbide for SMD devices to break through the limitation of carbon black. Accordingly, the hold current per PTC area can increase to larger than 0.16 A/mm2, or up to 1 A/mm2. However, as the rapid advancement of the mobile communication, the mobile apparatuses are demanded to be lightweight, compact and more powerful. Therefore, larger operating current is needed and the hold current per PTC area of 1 A/mm2 is not enough for current PTC protection applications. The PTC devices have to be improved to obtain higher hold current per unit PTC area, so as to make PTC devices of larger current with smaller PTC area.